Detection of volatile organic compounds (VOCs), which are widely used in industrial processes and household products, is very important due to significant health hazards associated with these materials. VOCs are commonly detected using photo-ionization detectors (PIDs), suspended hot bead pellistors or heated metal oxide semiconductors having functionalized layers. The detection methodology using PIDs is based on high-energy (typically >10.5 eV) photon induced ion generation, while that using hot bead pellistors takes advantage of the exothermic reaction (from auto-ignition of the VOCs) to produce a change in resistance. Heated metal oxide based sensing (e.g., TiO2 or SnO2) also relies upon a change in resistance, but at a temperature below the auto-ignition temperature of the VOCs.
Unfortunately, these techniques suffer from the problem of high power requirement as well as poor selectivity among VOCs, which is often important for proper identification of the source of a problem. Although the heated metal oxide method requires somewhat lower operational power, it involves complicated metal oxide functionalization processes.
Microcantilevers offer excellent opportunities for molecular sensing that arise out of their high sensitivity to various physical parameter changes induced by the analyte molecules. Microcantilever heaters, which are extremely sensitive to changes in thermal parameters, have been widely utilized for calorimetry, thermal nanotopography, and thermal conductivity measurements. Due to the small area of the microcantilever that needs to be heated (e.g., the tip of a triangular microcantilever), they also offer the possibility of reduced power consumption for high temperature operation. However, achieving repeatable and reliable functionalization of a microcantilever, especially over a small area, is a challenge that has thwarted practical applications of microcantilever-based sensors. On the other hand, unfunctionalized microcantilevers (typically made of silicon) are not particularly sensitive toward specific analytes, and are generally accepted to be incapable of performing selective detection. Thus, only a handful of studies utilizing uncoated microcantilevers to perform unique molecular detection have been reported. In these studies, detection has generally been based on changes in physical properties of the media surrounding the cantilever (i.e. viscosity, thermal conductivity, or changes in the analyte (i.e. deflagration temperature). Unfortunately, previous techniques are applicable only to a few specific analytes, and selective detection still remains a major challenge, especially when the analytes are diluted, are present in minute quantities or have similar physical properties as is the case for VOCs.
What are needed in the art are systems and methods that can quickly and efficiently identify different VOC's.